Increasing energy needs have focused attention on solid fossil fuels due to their availability in the United States in a relatively abundant supply and their potential value when converted into more useful forms of energy and feedstock. Coal is known to be a potential valuable source of chemical compounds as well, and considerable effort has been expended in attempts to develop a process for the efficient production of such chemicals and such fuel products.
It has been proposed to hydrogenate coal with hydrogen gas in the presence of a solvent and a catalyst at moderate to severe conditions of temperature and pressure. The product is determined by the reaction conditions, catalyst, and space velocity or residence time. It has been widely accepted that coal does not begin to decompose until it has reached the temperature of about 350.degree. to about 400.degree. C. At the temperatures generally employed for such hydrogenation, hitherto generally above 400.degree. C., the coal substance breaks down, the molecular chains in the coal being cleaved to from lower molecular weight substances. These products often have a molecular size such that they are suitable for use as fuel oils or the like, and they can be subjected to hydrocracking for conversion into "synthetic gasoline."
In the state-of-the-art hydrogenation processes, a recyclable "pasting oil" was necessary to initially dissolve or slurry the raw coal. The slurry of coal, usually containing a catalyst, was generally heated in the presence of hydrogen gas at about 450.degree. C. to 550.degree. C. and about 2,000 to 10,000 psig. Following hydrogenation finely-divided unreacted coal ash had to be filtered or otherwise removed from the heavy, viscous primary oil product. These processes, generally were not commercially acceptable because of the large capital investment, the high operating costs and the hydrogen requirements were too high in comparison with the value of the products obtained.
More recently, dry hydrogenation processes have been developed wherein coal is heated with hydrogen gas. However, these processes are generally batch-type processes and because they are conducted at greatly elevated temperatures and pressures, result in the production of hydrocarbon gases and liquids useful mainly as fuel. Greatly elevated temperatures and pressures at which these processes functioned make them difficult to operate and impractical.